The present invention relates generally to centrifugal blowers and more particularly to a means for reducing the noise of a centrifugal blower.
Centrifugal blowers and centrifugal fans are well known devices for blowing air and, in some instances, other fluids. A centrifugal blower has a fan wheel and a casing, or housing, with a cutoff as well as an air inlet and an air outlet. The fan wheel is of generally cylindrical configuration having blades facing forward or backward relative to the direction of rotation thereof about the axis of the cylinder. The casing typically is spiral in shape to collect the air delivered from the fan wheel and to conduct the same in a spiral flow pattern to the outlet.
A typical centrifugal blower includes a cylindrical fan wheel having a radial center along its axis and a spiral shaped blower housing with a blower outlet at one end thereof. The air inlet to the blower is in a side wall to permit air flow into the center of the fan wheel.
The spiral blower housing has an approximate center of curvature, otherwise known as a radial center, which is the theoretical center of the spiral, but approximately falling in the vicinity of the radial center of the fan wheel. The blower housing has two wall portions which lead generally to the blower outlet. One of those wall portions is that part of the spiral shape of the blower housing that is relatively far or remote from the spiral center, and the other wall portion may be considered the involute wall portion. The latter is generally curved inwardly along the track of the spiral toward the center thereof. Typically, the fan wheel is positioned so that it is relatively near the involute wall surface but relatively far from the remote wall portion. With the fan wheel so positioned, there is a generally annular spirally expanding flow path along which air may be blown by the rotating fan toward the blower outlet for discharge from the fan wheel in a generally linear flow direction or flow path. Linear is used herein to indicate a non-spirally confined flow path, and may be a divergent one as the air blown through the blower outlet may diverge or expand upon so leaving.
In the past, furnace parts were made of galvanized metal and the housings for furnace fans and other centrifugal blowers were also made from galvanized metal. In more recent technology, the housing for centrifugal fans is molded of a plastic material. This permits the integral formation of the housing with the outlet.
A "cutoff" occurs at the end of the involute wall portion relatively proximate to the blower outlet. A cutoff is formed at the transition point where the spiral air flow occurring in the housing is transformed to the relatively straight line discharge air flow through the blower outlet. During such transition, the typical cutoff tends to cut off or to impede air flow through the clearance area between the cutoff and the fan wheel, which in the instance of the present invention, rotates in a clock-wise direction.
Generally, the blower outlet is a cylindrical tube connected to the housing into which the air is blown from the housing through an opening into the cylindrical outlet. Integrally molded housings with outlets permit formation of openings which can optimize the path of the spiral air flow during its transition to the straight line air flow. The cutoff region contributes significantly to audible noise, notably, an audible tone or whistle-like sound. In spite of efficiently designed openings from the housing into the outlet, the noise level at the cutoff continues to exist.
Certain attempts have been made to deal with the audible noise at the cutoff. For instance, in U.S. Pat. No. 5,040,943, the passageway from the housing to the outlet is designed such that the opening is absent any vertical edge in juxtaposition to the fan, the edge having a geometry of uniformly changing dimensions. Whereas, such a design may have diminished some of the audible noise, there is still existing a considerable audible noise at the point where the blade of the fan passes the cutoff.
The present invention provides an improved configuration of the opening for the passage of air from the housing into the outlet in a centrifugal blower.